When the mirror returns a error, it was trying to interpret the error
message (e.g. <html><title>Internal server error..) as a download link.
By using -f on curl we get an empty reply and an exit code, which, as
we run in set -e mode, aborts.
Change-Id: Ibaa39aedb7db286f859c4b090114c6a233b150c7
421a0fa541